1. Field of the Invention
This invention relates to power capacitors and more particularly to means for retaining the capacitor's internals within its containment casing after rupture of the casing.
2. Description of the Prior Art
For more than 40 years polychlorinated biphenyl has been used as an impregnating fluid in power capacitors. In relatively recent years polychlorinated biphenyls were found to be environmentally undesirable in that their biodegradation rate was very slow and they tended to bioaccumulate in the environment, but especially in the fatty tissues of fish and mammals. After an exhaustive search for an acceptable replacement fluid for the polychlorinated biphenyls, isopropyl biphenyl fluid was found to have properties very similar to the polychlorinated biphenyls and was thus chosen for feature use in power capacitors manufactured by the assignee of the present invention.
Althoughisopropyl biphenyl is slightly more flammable than the polychlorinated biphenyls, isopropyl biphenyl fluid has a high resistitivity too fire as can be seen from its OSHA classification of III-3 combustible fluid. This is the highest OSHA classification for fluids and is similar to mineral oil which was generally used as the predecessor to the polychlorinated biphenyl fluids. Although the auto-ignition temperature of isopropyl biphenyl fluids is over 400.degree. C., it has occassionally caused capacitors internals to burn when the capacitor's containment casing ruptured and allowed the capacitor's internals to be completely expelled into the surrounding air.
Containment casing on capacitor units are typically metal whose joints are welded, brazed, or soldered. Such joints are the most vulnerable area on the capacitor casings to rupture due to increased gas pressure within the containment casing caused by internal arcing when the capacitor develops an internal fault. When the pressure exceeds the strength of the joint, the joint separates or ruptures and relieves the internal pressure. One approach for preventing the expulsion of the capacitor's internals and their subsequent ignition was to utilize a more strongly constructed containment casing by increasing the size of the joint's welds and/or the thickness of the containment casing. It was found that prevention of containment casing rupture required material and joint weld thicknesses so great as to be impractical. The most common rupture on capacitor units was found to be separation of the containment casing's bottom from the balance of the containment casing. A variety of bottom configurations were also tested in pursuit of a bottom to side weld joint which was stronger. For all attempts to make the capacitor units rupture-proof, it was found that the violence of the rupture increased as the strength of the joint increased, thus resulting in an undesirable design.
U.S. Pat. No. 3,600,636 which issued Aug. 17, 1971 and U.S. Pat. No. 3,634,798 which issued to Astleford on Jan. 11, 1972 and was assigned to the assignee of the present invention disclose bolts and rod-bracket assemblies respectively for supporting internal electrical assemblies from the cover of the surrounding electrical apparatus' structure. Such supporting devices are, however, unsuitable for containing parts from the supported electrical assembly within the structure. The previously mentioned supporting devices will retain the portions of the internal electrical assembly which remain intact and connected thereto, but will not retain those portions which are separated from the assembly during dissipation of the internal gas pressure through the rupture in the electrical apparatus' structure.
Valves and switches have also been the subject of experimentation in detecting pressure increases within the capacitor and relieving them or shutting down the capacitor prior to their buildup beyond the strength of the rupturable joint. Pressure relief valves or diaphragms and pressure switches were found to provide protection against case rupture only in low current tests when the pressure buildup from internal arcing was very slow. Additionally, the aforementioned monitoring and safety devices were judged undesirable due to their complexity and resulting malfunction susceptibility. In the worst case, such pressure relief valves can develop leaks that lead to capacitor degradation and failure. For such cases the pressure relief valves and other preventive systems would be considered a greater liability than an asset since auxiliary, safety devices such as these must be much more reliable than the device (capacitor in this case) that these systems are intended to protect.